1. Field of the Invention
The invention relates generally to metal halide lamps and more specifically to trichrominance lamps for use with twisted nematic subtractive light valves as are common in display systems such as projection television and computer monitor systems.
2. Description of the Prior Art
Display systems that use broad band light sources and red, green, and blue primary color filters so the individual colors can be modulated, are very inefficient. A better situation is for all the lamp power to be concentrated into three narrow bands at the primary colors. Filtering then wastes very little of the lamp's efficiency.
William H. Lake describes a selective spectral output metal halide lamp in U.S. Pat. No. 3,840,767, issued Oct. 8, 1974. Such lamp uses mercury, as is the norm in the prior art. A high pressure metal halide vapor arc lamp provides light concentrated in selected spectral bands for photochemical and reprographic applications. To produce light in the blue, green, and red bands, the lamp uses zinc iodide (ZnI.sub.2) as a buffer species whose radiation is largely suppressed. The emitter species include lithium iodide (LiI), thallium iodide (TlI) and gallium iodide (GaI.sub.3), and a small amount of mercury (Hg) that serves as a secondary buffer species.
Twisted nematic subtractive color light valves designed for primary color metal halide lamp illumination conventionally use notch polarizers rather than dichroic dye polarizers to select the red, green and blue peaks. Such primary color metal halide lamps render a substantially broader color gamut than is typical for miniature color displays. The broader color gamut is due to the enhanced spectral power distribution of the prime color lamp, wherein narrower primary color peaks, and a superior contrast and sharper polarization bands pass through the notch polarizers. The primary color lamps were designed to operate at 105 watts or one third the power of the existing xenon lamp. One third the power because the primary color metal halide lamp would produce the trichrominance spectral power distribution without filtering.
The use of mercury in metal halide lamps is ubiquitous. Mercury produces such a high vapor pressure compared to its alternatives, it is irresistible because its use results in high voltage operation, well behaved warm up characteristics and well behaved arc performance. But the mercury itself produces a strong yellow spectra band that requires filtering out in primary color projection systems. Thus, a primary color lamp that is intended to simplify filtering spoils its own principle advantages when it includes mercury. The prior art appears to be devoid of metal halide lamps that avoid the use of mercury.